Particle counters (often called particle sensors) are used to detect light scattered by particles entrained in a stream of fluid, e.g., in an air stream. Such counters draw air (with particles entrained therein) from a room, for example, and flow such air along a tube and through an illuminated sensor "view volume" to obtain information about the number and size of such particles. Such information results from an analysis of the very small amounts of light reflectively "scattered" by the particle as it moves through the view volume.
Such counters direct the air and accompanying particles through the view volume at a particular flow rate (often measured in cubic feet per minute) from one tube (inlet tube) across an open space (view volume) to another tube (outlet tube). In counters of this type, there is no tube wall (however transparent such wall might otherwise be) to impair light scattering and collecting. In other words, the particle is briefly illuminated by a very-small-diameter light beam as it "flies" through an open space.
Among other uses, particle sensors are used to obtain a measure of air quality by providing information as to the number and size of particles present in some specified volume of air, e.g., a cubic meter. Even work environments which appear to human observation to be clean--business offices, manufacturing facilities and the like--are likely to have substantial numbers of microscopic airborne particles. While such particles are not usually troublesome to the human occupants, they can create substantial problems in certain types of manufacturing operations.
For example, semiconductors and integrated chips are made in what are known as "clean rooms," the air in which is very well filtered. In fact, clean rooms are usually very slightly pressurized using extremely clean air so that particle-bearing air from the surrounding environs does not seep in.
And the trend in the semiconductor and integrated chip manufacturing industry is toward progressively smaller geometries. In turn, this requires that particle sensors be able to detect and assay smaller particles than theretofore might have been possible. Since particle characteristics are determined by the amount of light scattered thereby, an apparent solution to the problem of assaying smaller particles involves increasing the intensity of the light beam to increase the quantum of light reflected and scattered by very small and/or fast-moving particles.
In particle counters and sensors using a projected light beam as the light source, it is important to substantially fully attenuate or "kill" the light energy which is not scattered or reflected by a particle passing through the view volume. This is so since stray, "unkilled" light which finds its way to the opto-electrical system or back to the light source itself will degrade sensor performance.
One way to effect light beam attenuation (or "killing") is by using a light trap of the type disclosed in U.S. Pat. Nos. 5,467,189 (Kreikebaum et al.) and 5,731,875 (Chandler et al.). Such patents are incorporated herein by reference.
Such a light trap includes a primary surface or filter made of specially colored glass and having a bulk property. Most of the photon energy of the light beam is transformed into thermal energy in this surface. The light trap also usually includes a secondary surface having a highly light absorbing surface. Such secondary surface is positioned so that any residual light reflected from the primary surface (and there is usually very little of such light) impinges on the secondary surface.
The marketplace demands ever-greater performance of the products made by companies which use particle sensors of the type described above. As a consequence, designers and manufacturers of such particle sensors are being held to ever-higher demands for improved performance. For example, a way to detect very small particles, e.g., 0.1 micron and smaller, and/or to detect particles present in a relatively high air flow volume, e.g., 1 cubic foot per minute, through the sensor is to increase the power level of the light source. But since most of the photon energy emitted by the source must be killed (that is, very little is scattered by a particle), the primary filter or surface must dissipate increasing amounts of thermal energy.
A particle sensor and related method which address the matter of thermal energy dissipation would be an important advance in this field of technology.